In order to study the role of metal ions in the structure, regulation, and activity of proteases, we have expressed mutant trypsins in the methylotrophic yeast Pichia Pastoris. These mutant trypsins contain a designed metal site which involves the active site histidine residue. When metal is bound, the conformation of this residue changes, inactivating the enzyme. The mutants are based upon homologous serine proteases in the kallikrein family which naturally contains a metal site involving this active site histidine. Target residues were identified by sequence alignments of the kallikreins. Therefore, using the Computer Graphics Laboratory facilities, we have designed several mutants which contain potential metal sites. These trypsin mutants are inhibited by metal ions such as copper at nanomolar to micromolar concentrations, demonstrating that metalloregulation of proteases can be achieved. Some of this work has been published. Further improvements of this metal site through computer modelling is expected to enhance this ability.